1. Field of the Invention
The present invention relates generally to the field of semiconductor light emitting devices, and, more particularly, the present invention relates to a semiconductor light emitting device having a multi-layer structure formed of a nitride group III-V compound semiconductor.
2. Description of the Related Art
A nitride group III-V compound semiconductor, such as GaN, AlGaN, GalnN or the like, has a band gap width ranging from 1.8 eV to 6.2 eV, which makes it theoretically possible to realize a light emitting device which can emit red to ultra violet light. Therefore, such nitride group III-V compound semiconductors have recently attracted attention. When a light emitting device such as a light emitting diode (LED), a semiconductor laser diode (LD) or the like, is manufactured by using such a nitride group III-V compound semiconductor, it is necessary to form a multi-layer structure of, for example, semiconductor layers of GaN, AlGaN, GalnN or the like.
It is considered that a structure formed by substituting nitrogen for arsenic in a GaAs/AlGaAs system double hetero-junction structure (DH), can be practically employed and can be applied to the structure of a nitride laser diode. Nitride laser diode structures obtained by optimization or calculation, using GaAs/AlGaAs theory, have been reported. However, a sufficiently high luminous efficiency, i.e., oscillation of a carrier injection type light emitting device having such a structure, has not been achieved. In particular, a carrier injection type laser structure has not been reported yet.
It is believed by those skilled in the art that the following points are necessary steps for realizing oscillation of the carrier injection type laser, i.e., obtaining the sufficiently high luminous efficiency:
(1) to drastically reduce the defect density in the semiconductor layer from the present level of 10.sup.9 cm.sup.-3. PA1 (2) to establish an effective method of forming a reflective end surface of a laser resonator; and PA1 (3) to prevent the device from cracking because of the difference among lattice constants of layers in a multi-layer.
Practically, even if the above goals are achieved, it is impossible to obtain a satisfactory nitride light emitting device particularly a carrier injection type laser, which highlights existence of a serious problem. FIG. 8 shows a schematic, cross-sectional view of a light emitting device having a DH structure for optical pumping.
In this example, each layer of this light emitting device is formed by a metal organic chemical vapor deposition (MOCVD) method. A first buffer layer 2 made of GaN with a thickness of 30 nm is formed on a c-plane sapphire substrate 1 by low temperature growth. A second buffer layer 3 made of GaN with a thickness of 2 .mu.m is grown on the first buffer layer 2. Subsequently, a first cladding layer 4 made of Al.sub.0.13 Ga.sub.0.87 N with a thickness of 0.5 .mu.m, an active layer 5 made of GaN with a thickness d ranging from 0.01 .mu.m to 0.5 .mu.m (10 nm to 500 nm) and a second cladding layer 14 made of Al.sub.0.13 Ga.sub.0.87 N with a thickness 0.1 .mu.m are successively grown on the second buffer layer 3.
FIG. 9 is a graph showing a photoluminescence (PL) emission spectrum obtained when optical pumping in the active layer 5 with its thickness d=0.5 .mu.m of the light emitting device shown in FIG. 8 was carried out at a low temperature (4.2 K) with a He--Cd light source (a wavelength of 325 nm) for optical pumping with a power of 12 mW. In this emission spectrum graph, a PL emission peak having a wavelength of 334 nm is presented by light emitted mainly from the second AlGaN cladding layer 14 formed at the surface of the light emitting device and a PL peak having a wavelength of 356 nm is presented by light emitted from the GaN active layer 5. Because the GaN active layer 5 has an absorption coefficient of 10.sup.-5 cm.sup.-1 or greater, the optical pumping light does not reach the first cladding layer 4. Therefore, PL emission from the first cladding layer 4 is not observed.
A known method of increasing the luminous efficiency in a light emitting device, employing a GaAs/AlGaAs hetero-junction of a III-V compound semiconductor or a ZnSe/ZnCdSe hetero-junction of a III-VI compound semiconductor, is to make the thickness of an active layer thereof thinner. This method is practically employed when a quantum well laser is manufactured. This method of increasing the luminous efficiency utilizes the fact that if a width of a light emitting layer is smaller than a value which is twice a Bohr radius of an exciton, then a low dimension effect produced in such state leads to a change of a state density, increase of an exciton binding energy, increase of an emission transition probability, separation of a valence band and so on and consequently the emission efficiency is increased. In this case, the thickness of the active layer is usually set to be 100 .ANG. or smaller, while it is reported that the thickness is set to 20 .ANG..
It is considered that in principle this method is effectively employed for fabrication of a light emitting device made of a nitride compound semiconductor. Estimation based on an effective mass shows that when the thickness of an active layer of the nitride light emitting device is set to about 6 nm or smaller, a quantum well effect becomes effective (see Hiroshi Amano et al. Applied Electronic Physical Property Division Lett., vol. 1, No. 3, p25 (1995)). However, even if a light emitting layer of a nitride group III-V compound semiconductor having a hexagonal crystal system is made thinner so as to have a thickness twice of a Bohr radius of an exciton, it is impossible to increase the luminous intensity.
For increasing the luminous efficiency of light having a wavelength of 356 nm from an active layer, i.e., a light emitting layer of the above-mentioned nitride group III-V compound semiconductor light emitting device, it may be considered effective to make the light emitting layer thinner. However, the inventors have found that even if the thickness of the light emitting layer of the semiconductor light emitting device employing GaN/AlGaN hetero-junction or GaN/GaInN hetero-junction is set to 100 .ANG. or smaller, it is impossible to achieve such an increase of the luminous efficiency as is obtained in the light emitting device employing the GaAs/AlGaAs hetero-junction or ZnSe/ZnCdSe hetero-junction. To the contrary, it is observed that the luminous efficiency is lowered as the thickness of the light emitting layer is reduced.
Specifically, if the thickness d of the active layer 5 of the light emitting device having an arrangement shown in FIG. 8 is set thinner, e.g., set to d=100 nm, then, as shown in FIG. 10, a PL emission from the active layer 5 is drastically reduced to about 1/20 of that shown in FIG. 9. FIG. 11 is a graph showing a relationship between the thickness d of the active layer 5 and the luminous intensity. Study of FIG. 11 reveals that when the thickness d of the active layer is within the range of d&lt;50 nm, substantially, almost no light from the active layer is observed.
This phenomenon is considered to be a particular phenomenon of the nitride group III-V compound semiconductor light emitting device. The phenomenon may result from the fact that crystals of GaN group, such as GaN, AlGaN, GaInN or the like, have a hexagonal crystal system.
Through various experiments, studies and research, the inventors of the present application have reached the following realization. Specifically, the phenomenon that reduction in the thickness of the active layer lowers the luminous intensity in the light emitting device having a multi-layer structure formed of the above nitride group III-V compound semiconductor. For example, the semiconductor light emitting device, employing the above GaN/AlGaN hetero-junction, indicates the existence of a high-concentration non-radiative recombination centers on a surface of the hetero-junction. The existence of the high-concentration non-radiative recombination centers results from strain which is caused in an AlGaN cladding layer or a GaN active layer because of the difference between lattice constants of the AlGaN cladding layer and the GaN active layer in the nitride group III-V compound semiconductor light emitting device. However, since this phenomenon does not occur in a strain system compound semiconductor light emitting device such as a compound semiconductor light emitting device having a cubic crystal system employing an AlGaAs/InGaAs hetero-junction, it may be considered that this phenomenon is peculiar to the hexagonal crystal system.